Abstract
A striated muscle fiber is composed of a bundle of myofibrils in which sarcomeres run in series from one end of a myofibril to the other end. The sarcomere is composed of contractile proteins (actin and myosin), and structural proteins (Z-disc proteins, connectin/titin) together with many other proteins. The contractile force produced in each sarcomere is transmitted to the neighboring sarcomeres via Z-discs. Recently by applying nano-manipulation techniques (an atomic force microscope; AFM, optical tweezers, etc.), detailed mechanical properties of myofibrils of striated muscles have been studied. It was found that, by rupturing specific sarcomere components of myofibrils, the mechanical strength of myofibrils is marginal to sustain the force they produce. This suggests that various protein components are delicately involved in maintaining the molecular architecture of myofibrils intact against the force they produce. It is known that cardiac muscle is substantially stiff compared with skeletal muscle, possibly due to the difference in the content of connective tissues. In our recent study, the transverse stiffness of myofibrils of the two muscles was examined. Interestingly it was found that the transverse stiffness of cardiac myofibrils was significantly greater than that of skeletal myofibrils. This suggests that the transverse stiffness difference between skeletal and cardiac myofibrils may come from the difference in the physiological role of the two types of striated muscles. [J Physiol Sci. 2007;57 Suppl:S34]
